Stem cells are found in many organs of the adult human including bone marrow, peripheral blood, umbilical cord blood, spleen, tooth pulp, and brain. These progenitor cells are being investigated for their potential use as transplanted tissues in the treatment of diseases such as cancer, diabetes, stroke, amyotrophic lateral sclerosis (ALS) and Parkinson's disease. Little effort however is being directed toward enhancing the endogenous stem cells in the adult as an avenue to promote healing. In many of these diseases, and in aging, stem cells and progenitors are known to have a reduced proliferative activity. For example, neural stem cells, muscle satellite cells, and endothelial progenitors all show reduced proliferation in the aged and may play a role in pathology of age-associated diseases (Kuhn et al., 1996; Conboy et al., 2005; Dimmeler and Vasa-Nicotera, 2003). In cardiovascular disease, for example, there is a correlation between a reduction in peripheral blood endothelial progenitor cells and many risk factors for cardiovascular disease (Vasa et al., 2001; Hill et al., 2003). As many of the diseases being targeted by stem cell therapies are age-associated diseases, selecting nutritional strategies that increase stem cell proliferation in the aged population seems appropriate.
Hematopoietic stem cells (HSCs) have been investigated for many years for their utility in cancer treatments. Experimental investigations of hematopoiesis and clinical approaches to correcting its deficiencies have focused on cytokine activity. Cytokines modulate hematopoiesis by maintaining the self-renewal of stem cells and stimulating the proliferation and maturation of committed progenitor cells required for the continuous replacement of mature blood cells (Ogawa 1993; Socolovsky et al. 1998; Whetton and Spooncer 1998).
In vitro, various combinations of cytokines including interleukin-1 (IL-1), IL-3, IL-6, stem cell factor (SCF), and erythropoietin (EPO) have been found to support the growth of multipotent progenitor cells (Henschler et al. 1994; Miller and Eaves 1997). Individually, granulocyte-colony-stimulating factor (G-CSF) and EPO are growth factors for committed myeloid and erythroid progenitors, respectively (Demetri and Griffin 1991). Clinically, G-CSF and EPO provide effective treatments for neutropenia and anemia (Adamson and Eschbach 1990; Eschbach et al. 1990) and are used to enhance peripheral blood progenitors as an alternative to bone marrow transplantation for cancer patients. However, such treatments are costly, and are not without certain risks.
Decreases in hematopoietic and endothelial progenitors are associated with aging. Decreases in certain hematopoietic progenitors has been reported in frail aging women (Semba et al., 2005). Endothelial progenitor cells (EPC) are also derived from bone marrow and found in the circulating blood. Circulating EPC's home to sites of neovascularization and injury (Penn et al., 2004) and can then differentiate into mature endothelial cells (Asahara et al., 1999). Declines in EPC's are noted in patients with coronary artery disease (Vasa et al., 2001), and when isolated from patients with high risk factors for coronary artery disease, show increased senescence in vitro (Hill et al., 2003). It has been suggested that endothelial progenitors play a role in cardiovascular homeostasis and that the decline observed in aging and disease tips the balance toward injury rather than repair. Exercise has been shown to increase EPC's and this may be one of the reasons that exercise has beneficial effects on cardiovascular disease (Laufs et al., 2004). Developing nutritional based strategies to increase progenitors could push the balance back towards repair, thus having a significant impact on health.
Neural stem cells also decline with aging (Kuhn et al., 1996) and some have postulated that declines in neurogenesis with aging are related to cognitive decline while others disagree (Bizon et al., 2004; Drapeau et al., 2003; Prickaerts et al., 2004). Nonetheless, it has been shown that nutritional treatments, such as feeding with blueberry, which improve cognitive function (Joseph et al., 1999) also increase neurogenesis (Casadesus et al., 2004). Thus, there is a correlation between improved neural stem cell proliferation and improved cognitive function.
While potentially better treatments are currently in development, few research studies have investigated the effects of natural products, vitamins, and other nutrients which may modulate self-renewal of stem cells. However, in recent years there has been an upsurge of interest on the effects of various dietary insufficiencies on hematopoietic and immune responsiveness. Folate, vitamin 1312, and iron have crucial roles in erythropoiesis. Erythroblasts require folate and vitamin B12 for proliferation during their differentiation. Deficiency of folate or vitamin B12 inhibits purine and thymidylate syntheses, impairs DNA synthesis, and causes erythroblast apoptosis, resulting in anemia from ineffective erythropoiesis (Koury and Ponka, 2004). Other studies have recently found that dietary fatty acids, particularly oleic acid and linolenic acid, actively promote the proliferation of hematopoietic stem cells (Hisha et al., 1997; Hisha et al., 2002) as well as modulate the self-renewal of intestinal epithelial cells (Holehouse et al., 1998). Vitamin D has also received increasing attention over the past few years, in part, because recent studies suggest that nearly half the US population may be vitamin D deficient (Meyer, 2004). Recent laboratory studies demonstrate that vitamin D3 has a dramatic effect on stimulating the proliferation of various forms of multipotent progenitor cells, particularly those involved with the immune system (Mathieu et al., 2004). Recent laboratory research on cellular senescence (the end of the life cycle of dividing cells) suggests that the dietary nutrient, carnosine, found in muscle and brain of mammals, has the remarkable ability to rejuvenate cells approaching senescence, restoring normal appearance and extending cellular life span (Hipkiss et al., 1998; Holliday and McFarland, 2000).
The use of fruits or vegetables has the benefit of providing a cocktail of many different phytochemicals with multiple actions including antioxidant and antiinflammatory effects and is one reason they have been extensively studied in the field of cancer biology. Other studies suggest dietary supplementation with foods high in antioxidants, such, as blueberries, can prevent and even reverse cellular and behavioral parameters that decline as a function of aging (Joseph et al., 1999; Gemma et al., 2002). For example, dietary supplementation with 2% blueberry extract has produced both neuroprotective and neurorestorative effects in aged animals, perhaps as a result of modulation of cell signaling cascades (Williams, Spencer et al. 2004). Furthermore, blueberry extract has been shown to increase neurogenesis in the aged rat brain (Casadesus, NSci Abstract, 2002). We have shown that feeding blueberries to aged rats increases the survival and growth of hippocampal grafts grown in the anterior chamber of the eye (Willis et al., 2005), demonstrating that nutritional supplementation can not only increase proliferation of tissues, but promote appropriate differentiation.
Green tea is a drink made from the steamed and dried leaves of the Camellia sinensis plant, a shrub native to Asia. Green tea has been widely consumed in Japan, China, and other Asian nations to promote good health for at least 3,000 years. Recently, scientists have begun to study it's health effects in animal, laboratory, and observational human studies. Although active compounds within green tea extract have been shown to inhibit the growth of a number of tumor cell lines, they do not effect the growth of normal cells at similar concentrations (Chen et al., 1998; Wang and Bachrach, 2002) and actually may provide cellular protection from aging (Song et al., 2002).
In light of such findings reviewed above, it appears that certain nutrients, vitamins, and flavonoids could have important roles in maintaining the self-renewal of stem cells and stimulating the proliferation and differentiation of committed progenitors required for the continuous replacement of mature cells in the blood, brain, and other tissues. Furthermore, it may be possible to use certain natural products, either alone or synergistically, for the treatment of conditions where the stem cell replacement appears warranted such as aging or diseases associated with aging. However, the amounts of such substances that have shown actual results in studies are impractical to implement as supplementation to an ordinary diet. Even if studies are correct about the value of these substances, consumption of sufficient quantities to substantially improve health is impractical.
Aged mammals, such as rats, dogs and humans, can improve age-related declines in motor abnormalities and cognitive abnormalities with dietary interventions that include foods with a high antioxidant capacity. Antioxidants work at the cellular level; therefore, it would be expected that benefits of antioxidants in one mammal would be mirrored in other mammals. Certain foods were identified on the basis of the ability to show antioxidant activity in vivo in mammals and in an in vitro assay. Hundreds of foods were examined using this assay (Cao et al., 1997) and several were chosen with high in vitro antioxidant activity for testing in vivo. For example, when 18 month old rats are fed a diet in which 2% of the diet is a blueberry extract, after 2 months on this diet, we observe a significant improvement in motor performance on a balance beam (Joseph et al., 1999). We also observed a significant improvement on a Morris water maze in rats fed a diet supplemented with large quantities of strawberry, blueberry or spinach (Joseph et al., 1999). These same animals also show improved dopamine release in the striatum. A spinach diet improves age-induced deficits in motor learning using either a rod running motor learning task or classical eye blink conditioning (Bickford et al., 2000; Cartford et al., 2002). Markers of inflammation, such as the pro-inflammatory cytokine TNFα are increased in the brains of PD patients (Mogi et al., 1996), and 30 days following 6-OHDA lesions. We have shown that these diets decrease markers of oxidative damage and pro-inflammatory cytokines (Gemma et al., 2002; Cartford et al., 2002), furthermore these changes are related to the foods antioxidant activity as foods such as cucumber which are low in antioxidant activity have no effect (Gemma et al., 2002). We have been examining these diets in an animal model of Parkinson's disease. We have preliminary data showing that the blueberry or spirulina diet will increase the immune response 7 days following an insult and then prevent the prolonged activation of microglia at later time points. It is this later prolonged activation which we hypothesize is detrimental and reflects the ongoing inflammation observed in Parkinson's disease.
While benefits are known for incorporating antioxidants into the diet of humans, adjusting diets to incorporate a large proportion of these foods is difficult and often fails to incorporate sufficient amounts of antioxidants to make a significant difference on proliferation and differentiation of bone marrow cells, CD34+ HSCs, CD133+ progenitor cells from peripheral blood or any other stem cells. It would be of great benefit to identify certain natural compounds that can promote proliferation of hematopoietic stem cells or other stem cells, synergistically, such that the natural compounds could be taken in the form of a supplement that would have a significant, measurable effect.